I think you should spend less time thinking about skids for your own sanity. You live on a different continent and don't see or talk to skids. What they resent or not resent is not important. I have one insane adult SD and one good one, adult as well. I don't even spend much time thinking about my nice SD and i have good relationship with her, but I sure never think of a crazy one. Don't worry about what SS says or does. Not like you have to see him every day. Take focus off them. You have two kids to raise and educate and you mentioned one needs help to get caught up to be ready for school. So spend time doing that.
 

How DH raised his kids is irrelevant. He isn't even raising the ones he has with you as he doesn't live with you most of the time. So do what's important for your kids and don't worry about other stuff. You can't change your skids and your DH, they are who they are 

It would be interesting to know how your husband interacted with his older kids when they were children.  If he was hands off/distant, then I can understand why they are jealous that he's involved and cuddle with the younger kids.  Other than the money your husband constantly tosses at his kids, you never post about what he did to cultivate meaningful relationships with them.

SMs have posted about being bothered that their husbands treated BM one way but is the complete opposite with them.  I think it's normal for bio kids to experience the same feelings and frustrations.

Its funny how people think that a person remains the same no matter who they are married to.  People act different when they are married to another person.  Skids might be jealous because second wife brings out a better side in dear old dad than BM did.  There's a reason they got divorced.  Time for dad to explain that difference.  

You've been here on ST for over a year, and there has been no improvement in your situation. Why do you think that is?

You've made it crystal clear that your H's exwife is a piece of excrement. It's obvious both she and your H have made a lot of parenting mistakes. But you need to stop allowing yourself to dwell in anger and resentment over past events and people you cannot control. It's eating you up, and unless changes are made, it's going to ruin your marriage. What you need is tools and solutions for the problems in your marriage.

I know you're insanely busy, but I think you should make the time to find a marriage therapist there in Oz. One who does video or facetime counselling who can listen, support, and offer suggestions and strategies that will get you and your H working together to make your marriage a peaceful one.

It's pretty clear that your H doesn't know how to manage his baggage, draw boundaries, or parent his adult children while coping with a toxic ex. Communication is everything, especially when you're thousands of miles apart, and a counselor can help you both with that, too.

It seems like you both are stuck, with this skid issue between you. You need to identify what specific things you'd like your H to do, and what you will and will not accept. A professional can help you get past your emotions, distill these things, and express them to your H.

Sounds like you have it all figured out. So why are you so angry?

I really think you should give serious consideration to getting you and your H into counselling. I hear you that he's "not the type who'll see a therapist", but things aren't going to get better unless he learns some new ways. In fact, they're going to get worse because the resentment you're struggling with is caustic, and will eventually poison your marriage.

Whatever you decide, I hope you try to find a way to stop dwelling on your H's ex and the skids. You sound ever more unhappy and frustrated than you were when you first found ST, even with the new house.

Try reading the book The Language Of Letting Go by Melody Beattie. I found it helpful. You can't change your H or his baggage, but you can change your thoughts and philosophy on them.

You have a lot on your plate and are under a lot of stresst. I thknk it's time you told your DH that you don't want to hear anything (ANYTHING!) about your stepkids or their family. Most of what he tells you isn't important to your life (it's just a dose of negativity) and is entirely irrelevant to the situation (e.g. his response to your explanation of why you can't fly out). Youi don't want to hear another word about his first brood unless it is entirely relevant to your or your children. All he's doing is ramping up your stress - sure, it may be alleviating his but it's bogging you down in thoughts you don't need to dwell on. 

You can't change or control other people. You can't demand that Skids change their ways. And why do you even care? If your DH doesn't want to see a therapist you should see one yourself. You got to get this anger under control and have peace of mind